I have 2 NXS scopes. One is on a MPA 6.5 CM and the other is on a Barrett 98B 300 win mag. Both zero'd at 100 yds. Both on 20 MOA rail.

With the MPA, I have about 45 MOA of elevation. With the Barrett, it's tops out at 37 MOA. I have yet to swap the scopes to see if I get same results, but it seems odd there's so much difference between the two. At 100 yds, there shouldn't be that much drop between the 2 cartridges which would result in 8 MOA difference, or should it?

I've had the scope on the Barrett looked at by NF twice and they've assured me it's fine.

Imagine two identical right triangles. You change the length of one side of one triangle. Now they are different. Simple as that. You have different geometry because you have: different guns, different height over the bore axis, different caliber, different velocity. Makes perfect sense. If you swap the scopes, you'll need to re-zero them both.

I don't think that it is a big deal. 8 MOA is not a huge shift from one rifle to another. There are differences from rifle to rifle and barrel to barrel. I shoot with a rifle that changes cartridges and swaps barrels and bolts. All barrels have some curvature and the run out for the barrel is generally indexed to 12 o'clock when the barrel is fitted. That way, you'll get some vertical dispersion among the different barrels, but there will be very low horizontal dispersion. So, this known offset in vertical dispersion is programmed into your starting points on the ballistic calculator.

You are confusing the zero as mounted on those respective guns, not mechanical zero. There is a difference. That's why to zero a scope, you have to fire a few shots; you can't just turn the dials to the middle and call it good.

Like octavist said, there's different geometry between those 2 guns. The angle of the receiver, height over boreline, the alignment of the rings, any of those factors could contribute to the 0.133 degree (8 moa) difference.

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"There is no "best." If there was, everyone here would own that one, and no other." - DSB

Barrett zero-gap rings. I have an American Defense 20 MOA mount so I can try that, though it might be tight with a 56mm objective lens. It's pretty tight with existing rings so not sure if the 20 MOA mount will work.